Then a truck laden with explosives drove into the boundary wall, detonated its load and almost completely destroyed the structure.
A witness said the blast had left a crater 3m (10ft) wide.

TV footage showed bloodied victims being taken away on stretchers.""Over a dozen militants tried to storm the building,"" a police official who was inside the building told the BBC.

""An exchange of fire took place for at least 15 minutes. We then saw the pick-up truck trying to ram its way inside.""

A government spokeswoman, Sharmilla Farooqi, said: ""There are five policemen among the dead.""We have reports that there may be some women police among the casualties because there was a women's police station inside the building.""

One witness told the BBC that he had heard the exchange of gunfire before the explosion.

""I was playing tennis across the road at the Karachi Club when I heard gunshots and then a huge blast,"" said Ali Zaidi. "